1. Field of the Invention
The present invention relates to a process for producing a hollow molded article made of a thermoplastic resin. A thermoplastic resin is, hereinafter, referred to as a "resin".
2. Description of the Related Art
Heretofore, there are known a variety of methods for producing a resin molded article such us, for example, an injection molding, a compression molding and an injection-compression molding. Also, various applied techniques thereof have been developed. One of them is a method, so-called a gas-injection molding, wherein a high-pressure gas is injected into a resin being in a molten state in a cavity to form a hollow in the resin. Until now, a variety of modified gas-injection molding methods have been proposed.
JP-A-7-137080 discloses a method wherein a molten resin is supplied into a cavity, then a compressed fluid is injected into and ejected from the resin, and subsequently an opening, which doubles as an inlet and an outlet for the compressed fluid, is closed with a small amount of molten resin.
JP-A-2-295714 discloses a method wherein a mold having a cavity and a sub-cavity that is connected with the cavity is provided, next the cavity is filled with a resin, then a compressed fluid is injected into the resin, and subsequently a hollow is formed in the resin while ejecting a part of the resin to the sub-cavity.
JP-A-6-254865 discloses a method wherein a compressed gas is applied to a resin, which was injected into a cavity, to form a void, and the void is grown to form a hollow in the resin.
These methods have a purpose to prevent a thick portion of the molded article from sinking due to local internal shrinkage of the molten resin caused by its solidification. They successfully dispel sinking by providing a hollow in the thick portion. They, however, are required to use the compressed fluid.
JP-A-57-91241 discloses a method wherein a cavity is filled, with a high pressure, with a resin-that contains a foaming agent, at the same time when filling the cavity is completed a screw is retreated to reduce the pressure in the cavity, and then an unsolidified center portion of the resin is moved by a pressure caused by a decomposed gas to form a hollow in the resin.
JP-B-59-383 discloses a method for producing a hollow molded article wherein a synthetic resin is injected into a cavity, subsequently a fluid is injected into the synthetic resin, and then the fluid is ejected from the synthetic resin.